1. Field of the Invention
The present invention concerns a device for the control of the threads on a stitching machine. The output of modern stitching machines is so high that the control of the threads is extremely important. In contrast to the knitting machine, in which a very large number of threads are used, the feed is relatively low and the failure of a thread through thread breakage in a stitching machine means the loss of the material. This is all the more serious in that the number of the stitching seams for certain products is tending to decrease and the material feed rate is becoming higher and higher. Modern stitching machines operate with outputs of an average of 30-80 m/hr. In the production of auto upholstery often only two or three seams are stitched, with a speed of 100 m/hr being achieved.
2. Description of the Prior Art
This problem has been recognized and a solution has also been proposed. In Swiss Patent CH-PS357.955 a proposal is made to loop the needle thread from a supply spool around a roller and pass it through a terminal eye of a spring-loaded current-conducting contact arm. If the thread tension decreases as the result of a thread break, the contact arm presses on a contact bar and closes a circuit, whereby a relay is activated and the machine shut off. This system has proven its worth for stitching patterns with a relatively large number of stitching seams. With the very simple stitching patterns, such as cushion trimmings and auto upholstery with only a few stitching seams, which have become more and more prevalent recently, it has been found that when using only a few threads, the thread loop around the roller which is required to provide the thread tension does not work. Rather, just from the inertia in the system, the threads are frequently broken. Hence, in the stitching of auto upholstery, despite the high working speed, the work is performed without automatic thread control.